1. Field of the Invention
The present invention relates to a heat exchanger, and more particularly to a shell and tube heat exchanger having improved resistance to fouling of the exchanger fluid and corrosion of the tubes.
2. Description of the Related Art
A shell and tube heat exchanger is a class of heat exchanger that is most commonly found in oil refineries and other large chemical processing plants. This type of heat exchanger comprises a shell, i.e., a large vessel, and a bundle of tubes inside the shell. The shell and tube heat exchanger is designed to allow two fluids of different starting temperatures to flow through it. A first fluid flows through the tubes (the tube side), while a second fluid flows in the shell (the shell side) but outside the tubes. Heat is transferred between the two fluids through the tube walls, either from tube side to shell side or vice versa. The fluids may be either liquids or gases on either the shell or the tube side. In order to transfer heat efficiently, a large heat transfer area is generally used, requiring many tubes, which are usually disposed horizontally inside the tank-like shell structure.
To achieve effective heat transfer between the fluids in the heat exchanger, it is a must to distribute shell side fluid to flow uniformly over the tube banks. Various methods are employed to distribute the shell side fluid uniformly. One such commonly used method employs a baffle plate placed in between the inlet and the tube bundle. This method is effective enough in distributing the shell side fluid more or less uniformly around the tube bundles, but seems to be accompanied by fouling and corrosion on the tubes in the region next to inlet baffle plate.
Due to fouling and corrosion, these heat exchangers experience severe external wall thinning of the tubes beyond allowable limits. For continuous operation of such heat exchangers, the tubes are replaced on a regular basis, causing the plant to shut down. Several attempts have been made to eliminate the problem by upgrading the tube metallurgy, but the problem has persisted.
During heat exchanger operations, fluid enters the shell side with high velocity. Kinetic energy of the fluid is reduced sharply upstream and downstream of the impingement plates due to sudden expansion and constriction of flow in the area of the inlet nozzles. It has been empirically determined that recirculation zones proximate to and behind the impingement plates cause the fluid to become stagnant. Moreover, the recirculation zones encourage impurities and particles carried by the fluid to foul, (deposit), and create active corrosion that causes a thinning process in the tube wall to the extent that the heat exchanger must be replaced when the walls are too thin for safe operations.
Thus, a shell and tube heat exchanger solving the aforementioned problems is desired.